Electromagnetic interference ("EMI") is gen erated by a variety of sources such as electric field coupling, conductive coupling and common impedance coupling. These sources, in turn, generate two distinct types of interference. The first type of interference is known as conductive interference because it is conducted in power and ground lines. The second type of interference is considered radiated interference because it results in radiated electromagnetic energy waves. There is a need for design and manufacturing improvements that reduce conductive interference. In particular, there is a need to reduce conductive interference generated by electronically commutated motors which are widely used in a variety of applications such as, for example, household appliances. Often, such motors are commutated by switching power converters, the general operation of which is known in the art. Such switching power converters typically comprise a plurality of solid state power switching devices connected to the motor winding phases for selectively applying a voltage to the winding. Parasitic capacitances (sometimes referred to as "stray capacitances") are an unwanted yet virtually inevitable byproduct of the fact that electric circuits are constructed from non-ideal components. As defined in the IEEE STANDARD DICTIONARY OF ELECTRICAL AND ELECTRONIC TERMS (3d Ed.), parasitic capacitance (stray capacitance) occurs in varying degrees due to the "proximity of component parts, wires, and grounds." In general, the value of the parasitic capacitance increases inversely with the physical distance between the component parts, wires or grounds. In other words, as the distance between two conductors decreases, the parasitic capacitance between those two conductors tends to increase.
In switching power converters, parasitic capacitances exist between the switching elements and ground. Further parasitic capacitances exist between the connections to the motor winding and ground (typically the motor frame itself is tied to ground potential). These parasitic capacitances can result in the generation of conductive EMI. For example, the excitation of motor windings by high speed power switches in an inverter drive allows high frequency currents to flow through these parasitic capacitances to ground. The high frequency current components are the result of the fast switching rate. As is known in the art, current through a capacitor is reflected by the equation: EQU i=Cdv/dt
where i represents current, C represents the magnitude of the capacitance, and dv/dt is the time rate of change in the applied voltage. As can be seen, higher switching rates and sharp changes in the applied voltage result in increased dv/dt, and thus, increased current flow i for a given value of capacitance C.
These circulating currents are known as "ground currents." One of the common techniques for reducing ground currents is to use EMI filters--typically a common mode choke and capacitors in series with the input power supply. Depending upon the particular application, several filter stages may be required to achieve the desired level of filtering. These filtering techniques, however, do not reduce the parasitic capacitances associated with the motor windings that actually cause the EMI. In other words, filters address the symptoms, not the source of the problem. Several approaches are known in the art for dealing with these parasitic capacitances. For example, the thickness and dielectric characteristics of the isolating material between the winding and stator may be altered to achieve the desired results. The drawbacks of this approach, aside from increased cost and complexity, are the risk of thermal degradation of the motor and possibly the motor controller itself.
There is a need, therefore, for a cost effective way to reduce conductive EMI in electronically commutated motors. In particular, there is a need to reduce the parasitic capacitances associated with motor windings without significantly impacting motor design or manufacturing costs.